The basic principles of operation and design of personal pagers are well known to those of skill in the art. In general, a pager contains a receiver which monitors a selected frequency or channel, and an alerting device, which alerts the user that some action needs to be taken. The alerting device may be a simple vibrator, a tone generator, an LED, or a display which indicates the telephone number to be called. Also, the pager may only operate on a single frequency or may be programmable to operate on a selected one of a predetermined set of frequencies.
It is desirable that pagers consume the minimum amount of power so that small batteries can be used and so that the batteries need to be infrequently changed. The alerting device is only activated in response to an incoming signal directed (addressed) to that pager and so the power drain of the alerting device is generally inconsequential. However, the power consumption of the receiver is significant. Therefore, in order to maximize battery life, it is necessary to minimize the power consumption of the receiver.
Furthermore, it is desired that the parts count and size be minimized so that the pager may be conveniently placed in a shirt pocket, hooked on a belt, attached to a purse, etc. Also, it is always desirable to minimize the cost of any device.
A direct conversion receiver uses a local oscillator and a mixer to produce a baseband output signal from an incoming signal. In these receivers the local oscillator frequency is the same as the frequency of the signal carrier. This type of direct conversion receiver is well known to those of skill in the art.
Because of the frequency used for paging signals, typically between 929 and 932 MHz, a frequency synthesizer is desirable for convenient selection of the monitored frequency or channel. Furthermore, to reduce the parts count, cost, and receiver size, a direct conversion receiver is desirable. However, a 930 MHz frequency synthesizer would consume a large amount of current, thereby reducing battery life.
Therefore, there is a need for a direct conversion receiver which minimizes parts count, cost, size, and battery current drain.
In addition, there is a need for a frequency synthesizer for a direct conversion receiver which has low power consumption, low parts count, frequency stability, and a programmable output frequency for channel selection.
Two-stage direct conversion receivers use two local oscillators and mixers to produce a baseband signal from the incoming signal. The first local oscillator produces a first oscillator signal that has a frequency that is different from the frequency of the carrier of the input signal. The first local oscillator signal and the incoming signal are mixed in the first mixer to produce an intermediate frequency (IF) signal. The second local oscillator produces a second local oscillator signal that has a frequency that is the same as the frequency of the carrier of the IF signal. The second local oscillator signal and the IF signal are mixed in the second mixer to produce a baseband output signal. This type of two-stage direct conversion receiver is also well known to those with skill in the art. The baseband output signal is then provided to a circuit which uses the signal, such as an amplifier, logic circuit, and/or alerting device.
However, for channel selection purposes, it is desired that the frequency of oscillation of one of the oscillators be selectable (frequency agile) and the frequency of oscillation of the other oscillator be fixed and highly stable. It is possible to use two crystal controlled oscillators to achieve this purpose, with one of the crystals providing a reference for frequency synthesis by a programmable oscillator. However, the component count and cost would be higher than desired. Also, a two-step calibration procedure would be required because each oscillator would have to be tuned independently.
Therefore, there is a need for a circuit that produces two non-harmonic oscillation frequencies from one crystal while maintaining frequency coherence between the two oscillations.
Furthermore, in a two-stage direct conversion receiver it is beneficial for the two oscillators to track each other to provide for easier tuning. Therefore, there is a need for an oscillator for a two-stage direct conversion receiver which has minimal parts count and provides for satisfactory frequency stability.